The design flexibility afforded by many thermoplastic resin compositions, their relative light weight and corrosion resistance make them attractive materials for many uses, including the replacement of metal components in many applications including motor and recreational vehicles, appliances, tools, electronics, furniture, and toys. However, in the construction, furniture, transport or electrical/electronic industries, thermoplastic resin compositions are preferably made flame retardant to promote product safety, prevent the spread of fire and reduce destruction of products exposed to fire. The conventional practice of imparting flame retardance to thermoplastic resin compositions has been the addition of one or more flame retardants or a flame retardant mixture, which typically include a halogenated organic compound such as brominated polystyrene as the flame retardant and an antimony compound as a synergist for the retardant.
However, halogenated flame retardants tend to decompose or degrade at the processing temperatures of thermoplastic resins, which implicates potential health and environmental effects due to the gases that are released. Consequently, there has been a trend away from using halogenated compounds or mixtures containing them to impart flame retardance.
Another conventional approach to impart flame retardance to thermoplastic resin compositions has been the addition of red phosphorus. Int'l. Pat. App. Pub. No. WO 92/20731 discloses a composition comprising an elastomer, a flame retardant comprising red phosphorus and ammonium polyphosphate as a flame retardant synergist. Moreover, the use of fine red phosphorus powder homogeneously dispersed in the resin is known and practiced. In addition to the hazards of fire and explosion related to handling fine red phosphorus powders, the very combustion of red phosphorus causes the emission of toxic fumes due to the formation of phosphine.
To avert the hazards of using halogenated flame retardants and red phosphorus, phosphinate salts, that is, salts of phosphinic acids, also known as phosphinates, have been substituted in thermoplastic resin compositions. DE Pat. Nos. 2,252,258 and 2,447,727 disclose phosphinates used as flame retardants. U.S. Pat. No. 4,180,495 discloses is the use of poly(metal phosphinate) salts in flame retardant polyesters and polyamides. U.S. Pat. No. 6,255,371 discloses flame retardant compositions comprising a) phosphinates, diphosphinates, or polymers of these and b) condensation products of melamine, reaction products of melamine with phosphoric acid, reaction products of condensation products of melamine with phosphoric acid and/or mixtures of these. U.S. Pat. No. 6,270,560 discloses salt mixtures made from aluminum phosphinates, aluminum hydroxide, aluminum phosphonates and/or aluminum phosphates suitable as flame retardants for polymeric molding compositions. U.S. Pat. Nos. 5,780,534 and 6,013,707 disclose flame retardant polyester compositions containing calcium or aluminum salts of phosphinic acid or disphosphinic acid.
A disadvantage of using halogen-free, flame retardant compositions is that, upon exposure to flame, such compositions emit a high level of smoke, which can cause smoke inhalation hazards severe enough to require evacuation of the workplace. Therefore, a need remains for halogen-free, flame retardant compositions comprising thermoplastic elastomers which exhibit the desired flame retardance as well as low smoke emission properties.